Magnetic detection devices generally rely on the phenomenon which results when a conducting material, such as a metallic object, is positioned in a magnetic field. Specifically this phenomenon involves inductance coupling between the detection device and the conducting material (object) whereby a changing current in one (the detection device) induces a current in the other (the object). The inducement in this case is caused by a magnetic field (B) which is generated by the changing current in the detection device. In response, a current is induced in the object which will alter the magnetic field (B). Magnetic detection devices are useful for finding hidden or concealed objects because the alterations in the magnetic field that result from inductance coupling are detectable.
Although metal detection devices are efficacious in many circumstances, it happens that a conducting material which is configured as a sheet (i.e. the sheet is flat) can not be so easily detected when it is oriented edgewise with its flat surfaces substantially parallel to the magnetic flux lines in a magnetic field. Further, even when flat conducting materials are oriented with their surfaces perpendicular to the magnetic flux lines, interference from other conductors can effectively prevent detection of the target material. Specifically, it is known that the loop-like metallic frames which are used for reinforcing large suitcases will cause substantial interference in a magnetic field. Consequently, a flat, sheet-shaped electrically conducting object which is carried in a reinforced suitcase will not be detected by magnetic detection techniques.
Inductance coupling is effectively nullified if there is zero mutual inductance (M=0). As a practical matter this will occur even when a conducting material is located in a magnetic field if the flux into the object (.PHI..sub.in) is equal to the flux that is coming out of the object (.PHI..sub.out). Stated differently, M=0 when, .PHI..sub.in =.PHI..sub.out. Under this condition, the object will not cause an alteration of the magnetic field. Thus, no detectable signals will be generated and the object will be effectively invisible.
In light of the above, it is an object of the present invention to provide a device and method for detecting random conducting objects which are located inside a conducting loop which generates a magnetic field that exhibits substantially zero mutual inductance with the loop and thereby effectively eliminates the effect of the loop during the detection of the objects inside the loop. It is another object of the present invention to provide a device and method for detecting random conducting objects that are located inside a conducting loop wherein the mutual inductance between the detecting magnetic field and the loop can be adjusted to zero. Still another object of the present invention is to provide a device and method for detecting random conducting objects located inside a conducting loop regardless whether the objects are sheet-like or have a more three dimensional volumetric shape. It is also an object of the present invention to provide a device for detecting random conducting objects located inside a conducting loop which is easy to use, relatively simple to manufacture, and comparatively cost effective.